The present invention relates to gallium arsenide integrated circuits, and more particularly to a method of fabrication of such circuits.
Integrated circuits are widely used in modern radar systems and electronic countermeasure systems. For higher frequency systems above about 5 GHZ, gallium arsenide devices are preferred to silicon devices because of the higher carrier velocity in gallium arsenide, and because it can be easily prepared as an insulating substrate for the formation of monolithic integrated circuits. Such circuits and fabrication techniques are set forth in, "Monolithic Microwave Amplifiers Formed by Ion Implantation into LEC Gallium Arsenide Substrates", IEEE Transactions On Electron Devices, Vol. ED 28, No. 2, February 1981.
Gallium arsenide integrated circuits are formed using Schottky barrier gate, field effect transistors. A plurality of transistor contacts are generally connected in electrical parallel to achieve reasonable power operation. With a plurality of transistor gates in parallel it is not possible to electrically interconnect the plural sources, drain and gates in a planar fashion without some means of crossing one interconnection over the others. In discrete devices, stitch bonding of small diameter wires has been used to make these parallel interconnections, but is extremely labor intensive and yields unreliable electrical contact. Such stitch bonded wires also add parasitic inductance to the circuit which can seriously degrade transistor gain.
For prior art discrete gallium arsenide devices, the substrate is typically very thin, only about 30 micrometers, and it was therefore possible to form apertures or vias through the substrate beneath a transistor source or drain contact. An electrical interconnect could then be made through the via to eliminate the need for the stitch wire bond interconnect. This via formation in very thin gallium arsenide substrates was by a single stage wet chemical etch process, as generally set forth in copending application Ser. No. 292,977, filed Aug. 14, 1981, entitled "Wet Chemical Etching of III/V Semiconductor Material Without Gas Evaluation", owned by the assignee of the present invention.
The gallium arsenide substrate thickness for producing monolithic microwave amplifiers and other such integrated circuits must typically be about 100 micrometers thick. This thickness is necessary to keep circuit losses acceptably low, while maintaining satisfactory heat-transfer characteristics for the power field effect transistors. It had not been found possible to fabricate interconnection vias through such thicker substrates, i.e. 100 micrometers, without the vias become too large in diameter and exceeding the area of the transistor contact with which the via is aligned.